1. Field of the Invention
The present invention relates to an optical modulator, and more particularly, to an optical SSB modulator that generates a modulation signal having a suppressed single side band.
2. Background Art
With the increased use of radio frequency resources in the microwave frequency band, frequencies used for various kinds of next-generation wireless systems have become higher and higher. However, such transference of wireless systems to higher frequency bands makes the coverage of one cell narrower, produces a blind area caused by a building shadow and the like due to the linearity of electromagnetic waves, and makes it difficult for electromagnetic waves to enter indoors.
Under such circumstances, a so-called optical fiber wireless system in which a radio signal is converted to an optical signal, and the optical signal is transmitted to a desired site with low loss through an optical fiber and is then again converted to electromagnetic waves at the site for wireless delivery has become increasingly important.
In such an optical fiber wireless system, since radio signals having different frequencies are transmitted through the optical fiber, there is a need for optical modulation over a wide range of frequencies (for example, see Japanese Unexamined Patent Application, First Publication No. 2005-274806). In addition, it has been considered that a SSB (Single Side Band) modulator that generates a modulation signal having a suppressed single side band could be used to prevent a signal from deteriorating due to dispersion of an optical fiber transmission line.
However, since a conventional SSB modulator performs SSB modulation with a wideband signal, which is a combination of an RF carrier and a data signal, it requires a wideband signal source.
In addition, it is commonly known that the modulation efficiency of a SSB modulator deteriorates in a high frequency domain. For the purpose of preventing such deterioration of the modulation efficiency, a modulator using a resonance electrode structure has been proposed which is effective in increasing modulation efficiency at a specific frequency. However, in the proposed modulator, an RF carrier frequency depends on the modulator structure. Accordingly, a plurality of modulators having different structures depending on RF carriers is required to be arranged.
In addition, a conventional SSB modulation system for adjusting a ratio of the carrier power to signal power in order to optimize a CN (carrier to noise) ratio after receipt (for example, see “Experimental study on an optimal optical modulation index in a millimeter wave self-heterodyne/optical heterodyne detection-based digital terrestrial delivery system, Hashimoto et al., The Institute of Electronics, Information and Communication Engineers (IEICE) symposium, 2005”) also cannot obtain sufficient characteristics in a high frequency domain.